An embodiment of the present invention relates generally to a vehicle safety seat system, and more particularly, to a dual stage variable load energy absorber for preventing injuries to occupants in vehicles during mine blasts or the like for both blast and slam down crash phases.
During the 1980's, energy absorbers (EAs) were developed and extensively tested for certain rotary wing aircraft (e.g., helicopters). The purpose of these devices was to limit the vertical loading (accelerations) experienced by the occupants, and thus reduce the probability of serious injuries that could occur during certain types of crash events. As a result of this development, unique and discrete EAs were ultimately integrated into several series of U.S. Military helicopters and, more recently, select ground vehicles, and these devices have been successfully used for decades to reduce the probability of crash induced spinal injuries.
Helicopter seat mounted EA devices and current ground vehicle EA devices only operate to mitigate a single loading event over a well defined range of exposures. When a military type ground vehicle is exposed to a blast, the occupant experiences two defined exposures within a short time period: loading from the blast itself as the vehicle is being propelled upward and when the vehicle returns to the earth, which produces a “slam down.” The force exposure of the slam down can be just as severe as the force exposure of the blast portion. Thus, strict adaptations of the helicopter EA devices (currently in use in select ground vehicles) are not sufficiently effective. Prior attempts to adapt the helicopter EA devices create additional hazards for the ground vehicle occupant without completely mitigating the exposure experienced by the occupant.
The function of an EA is to permit the seat to stroke downward in a controlled manner to reduce the crash loads and accelerations applied to the occupant, (i.e., limit acceleration forces applied to the seated occupant) as compared to the crash acceleration input at the floor of the vehicle. The stroking is designed specifically to initiate at a predetermined force level and continue until the input load drops below the threshold of human injury tolerance. The available stroke distance must be sufficiently long to avoid expending the stroke distance and instantaneously acquiring the velocity of the vehicle floor. The basis for the theory behind this concept is the law of “conservation of energy,” which manages the energy of a crash through reshaping the occupant's acceleration versus time response curve in a manner that reduces the peak accelerations. Adaptations of helicopter EA devices for use in Military Ground Vehicles do not account for the dual effect of the blast and slam down, nor do they account for the variation in load created by the varying weights of the occupant, both with and without battle gear, or the variable sizes of blasts experienced by military vehicles.
In a Military Ground Vehicle, the effect of any given blast can have a large variability due to the variability in occupant mass, but is also exacerbated by the point of blast contact with the vehicle, the magnitude of the blast (blast energy), the type of blast, and the vehicle's characteristics, such as mass and deformation. This variability poses a unique threat in that the input shock effect and input acceleration to the occupants can vary widely in both magnitude and direction. Unlike ground vehicles exposed to a large variety of mine and Improvised Explosive Device (IED) blasts, the vertical input accelerations imposed during helicopter crashes are relatively predictable and fall within a known, and relatively narrow, band. One reason is that many survivable helicopter crashes tend to occur as a result of power loss followed by autorotation where the aircraft impact velocity vector is well established both in magnitude and direction. In contrast, a ground vehicle exposed to a blast experiences the wide range of input accelerations and orientations due to the blast itself, followed by the input accelerations resulting from the subsequent slam down when the vehicle returns to the ground. The slam down acceleration vectors also vary greatly due to the blast severity, the orientation of the vehicle when it impacts the ground, and the ground characteristics.
It is therefore desirable to provide an EA device for a military-type ground vehicle that accounts automatically for the variation in total occupant seated mass; provides energy absorption for the variable blast phase; provides energy absorption for the slam down phase by automatically resetting itself after the blast phase; prevents “bottoming out” during either of the loading phases so as to not generate a dynamic amplification spike to the occupant; and maintains crash effectiveness in either frontal, lateral, rear or rollover crash events.